1. Field of the Invention
The present invention relates to a capacitor having a dielectric layer and a method of manufacturing the same. More particularly, the present invention relates to a capacitor having a dielectric layer that includes a composite oxide, the composite oxide including a transition metal and including a lanthanide group element, which may reduce leakage current, and a method of manufacturing the same.
2. Description of the Related Art
As technology has developed, lightweight and miniature electronic devices with high performance have drawn increasing attention. To make electronic devices, e.g., mobile phones, MP3 players, digital cameras, PDAs, etc. smaller, continuing miniaturization, e.g., by increasing the level of integration, is required. One aspect of this ongoing miniaturization is the continuing development and increased integration of embedded memory devices. A ferroelectric random access memory (FRAM), which typically uses a dielectric material for an information storage layer, is a non-volatile memory device with applications in mobile devices. To realize a high level of integration of the dielectric memory device, it is important to increase the total capacity of ferroelectric capacitor per unit area. Many memory devices, e.g., dynamic random access memory (DRAM), rely on capacitors to store information. For a given dielectric material, the capacitance of a capacitor may be increased by increasing the area of the capacitor, as shown in Equation 1:
                    C        =                  ɛ          ⁢                                          ⁢                                    A              t                        .                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, ε is capacitance, c is a dielectric constant, A is an effective area, and t is the thickness of a dielectric layer.
As set forth in Equation 1, when the thickness of a dielectric layer is reduced and the effective area of a capacitor is increased, the capacitance of the capacitor increases. This poses a dilemma as the integration of semiconductor memory devices continues to increase and memory devices must become more dense. In particular, increasing the area of the capacitor may limit the increase in the integration density of a semiconductor device if the capacitor has a planar structure.
Another aspect of increasing the density of memory devices is that the required charge storage capacity of a capacitor (and therefore of the dielectric) per unit area needs to increase, since the required charge needs to be stored in a smaller capacitor. To increase the charge storage capacity of a dielectric, it is desirable to reduce the thickness of the dielectric layer and/or use a material having a high dielectric constant for the dielectric layer. However, when the thickness of the dielectric layer is reduced, the dielectric may increasingly lose its stored charge due to a significant increase in tunneling current. That is, there is a limitation in reducing the thickness of a dielectric layer, since the possibility of generating a leakage current increases when the thickness of the dielectric layer is excessively reduced. Accordingly, much effort has been directed to developing a material having a higher dielectric constant and better leakage current characteristics than a conventional dielectric. However, there are many difficulties associated with the development of such a new material.
FIG. 1 illustrates a cross-sectional view of a conventional capacitor having a dielectric layer. Referring to FIG. 1, a lower electrode 12, a dielectric layer 13, and an upper electrode 14 are sequentially formed on a lower structure 11. The dielectric layer 13 is generally formed of a dielectric material having a high dielectric constant. However, when a transition metal oxide, such as TiO2, is used for the dielectric layer 13, a larger leakage current is produced by the capacitor than when other materials are used for the dielectric layer 13.
FIG. 2A illustrates a graph of leakage current density versus physical thicknesses (in Å) of dielectric layers formed of different materials, measured over a range of operating voltages. Referring to FIG. 2A, all of the four materials have high leakage currents in a range of thickness from 40 to 60 Å, which constitutes a very thin dielectric. Further, TiO2 leads to a relatively greater leakage current than other materials.
FIG. 2B illustrates a graph of O2 concentration profile with respect to Ti oxides according to depths of the Ti oxides, as determined by X-ray photoelectron spectroscopy (XPS). Ruthenium (Ru) is used for upper and lower electrodes 12 and 14 in FIG. 1, and TiO2 is used for the dielectric layer 13. The results illustrated in FIG. 2B indicate that Ti exists in two oxide forms (3+ and 4+). Referring to FIG. 2B, TiO2 peaks are observed at different depths. In addition, as indicated by an arrow, the oxidation state of Ti changes and two oxide forms are apparent. Typically, when a transition metal oxide having a variety of oxidation states is used for a dielectric layer 13, it may be difficult to reduce leakage current.